arXiv subjects

Karsten Albe

Publications and source records attributed to Karsten Albe.

46 records · Page 3Linked to original sources

arXiv2016

Positive and negative electrocaloric effect in BaTiO$_3$ in the presence of defect dipoles

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO$_3$ is studied by means of lattice-based Monte-Carlo simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen vacancy-acceptor associates are described by fixed defect dipoles with orientation parallel or anti-parallel to the external field. By a combination of canonical and microcanoncial simulations the ECE is directly evaluated. Our results show that in the case of anti-parallel defect dipoles the ECE can be positive or negative depending on the density of defect dipoles. Moreover, a transition from a negative to positive ECE can be observed from a certain density of anti-parallel dipoles on when the external field increases. These transitions are due to the delicate interplay of internal and external fields, and are explained by the domain structure evolution and related field-induced entropy changes. The results are compared to those obtained by MD simulations employing an {\it{ab initio}} based effective Hamiltonian, and a good qualitative agreement is found. In addition, a novel electrocaloric cycle, which makes use of the negative ECE and defect dipoles, is proposed to enhance the cooling effect.

physics.comp-ph↗

arXiv2015

Anomalous compliance and early yielding of nanoporous gold

We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson-Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak.

cond-mat.mtrl-sci↗

arXiv2015

Solid-state amorphization of Cu nanolayers embedded in a Cu64Zr36 glass

Solid-state amorphization of crystalline copper nanolayers embedded in a Cu64Zr36 metallic glass is studied by molecular dynamics simulations for different orientations of the crystalline layer. We show that solid-state amorphization is driven by a reduction of interface energy, which compensates the bulk excess energy of the amorphous nanolayer with respect to the crystalline phase up to a critical layer thickness. A simple thermodynamic model is derived, which describes the simulation results in terms of orientation-dependent interface energies. Detailed analysis reveals the structure of the amorphous nanolayer and allows a comparison to a quenched copper melt, providing further insights into the origin of excess and interface energy.

cond-mat.mtrl-sci↗

arXiv2014

Deformation Induced Complexion Transitions in Nanocrystalline Alloys

Grain boundary (GB) enthalpies in nanocrystalline (NC) $\mathrm {Pd_{90}Au_{10}}$ are studied after preparation, thermal relaxation and plastic deformation. By comparing results from atomistic computer simulations and calorimetry, we show that increasing plastic deformation of equilibrated NC $\mathrm {Pd_{90}Au_{10}}$ specimen causes an increase of the stored GB enthalpy $Δγ$. We interpret this change of $Δγ$ as stress-induced complexion transition from a low-energy to a high-energy GB-core state. In fact, GBs behave not only as mere sinks and sources of zero- and one-dimensional defects or act as migration barriers to the latter but also have the capability of storing deformation history through configurational changes of their core structure and hence GB enthalpy. Such a scenario can be understood as a continuous complexion transition under non-equilibrium conditions, which is related to hysteresis effects under loading-unloading conditions.

cond-mat.mtrl-sci↗

arXiv2014

Low temperature heat capacity of severely deformed metallic glass

The low temperature heat capacity of amorphous materials reveals a low-frequency enhancement (boson peak) of the vibrational density of states, as compared with the Debye law. By measuring the low-temperature heat capacity of a Zr-based bulk metallic glass relative to a crystalline reference state, we show that the heat capacity of the glass is strongly enhanced after severe plastic deformation by high-pressure torsion, while subsequent thermal annealing at elevated temperatures leads to a significant reduction. The detailed analysis of corresponding molecular dynamics simulations of an amorphous Zr-Cu glass shows that the change in heat capacity is primarily due to enhanced low-frequency modes within the shear band region.

cond-mat.mtrl-sci↗

arXiv2013

Formation and switching of defect dipoles in acceptor doped lead titanate: A kinetic model based on first-principles calculations

The formation and field-induced switching of defect dipoles in acceptor doped lead titanate is described by a kinetic model representing an extension of the well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)]. Energy barriers for defect association and reorientation of oxygen vacancy-dopant (Cu and Fe) complexes are obtained from first-principles calculations and serve as input data for the kinetic coefficients in the rate equation model. The numerical solution of the model describes the time evolution of the oxygen vacancy distribution at different temperatures and dopant concentrations in the presence or absence of an alternating external field. We predict the characteristic time scale for the alignment of all defect dipoles with the spontanenous polarization of the surrounding matrix. In this state the defect dipoles act as obstacles for domain wall motion and contribute to the experimentally observed aging. Under cycling conditions the fully aligned configuration is perturbed and a dynamic equilibrium is established with defect dipoles in parallel and anti-parallel orientation relative to the spontaneous polarization. This process can be related to the deaging behavior of piezoelectric ceramics.

cond-mat.mtrl-sci↗

arXiv2012

Antisite traps and metastable defects in Cu(In,Ga)Se2 thin-film solar cells studied by screened-exchange hybrid density functional theory

Electronic structure calculations within screened-exchange hybrid density functional theory show that Cu(In,Ga) antisites in both CuInSe2 and CuGaSe2 are localized hole traps, which can be attributed to the experimentally observed N2 level. In contrast, GaCu antisites and their defect complexes with copper vacancies exhibit an electron trap level, which can limit the open-circuit voltage and efficiency in Ga-rich Cu(In,Ga)Se2 alloys. Low-temperature photoluminescence measurements in CuGaSe2 thin-film solar cells show a free-to-bound transition at an energy of 1.48 eV, in very good agreement with the calculated transition energy for the GaCu antisite. Since the intrinsic DX center does not exhibit a pinning level within the band gap of CuInSe2, metastable DX behaviour can only be expected for GaCu antisites.

cond-mat.mtrl-sci↗

arXiv2012

Thermodynamics of mono and di-vacancies in barium titanate

The thermodynamic and kinetic properties of mono and di-vacancy defects in cubic (para-electric) barium titanate are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which under metal-rich (oxygen-rich) conditions all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. Since oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono and di-vacancies influence the initial defect distribution in the ferroelectric phases and therefore the conditions for aging.

cond-mat.mtrl-sci↗

arXiv2012

Modeling the electrical conductivity in BaTiO3 on the basis of first-principles calculations

The dependence of the electrical conductivity on the oxygen partial pressure is calculated for the prototypical perovskite $\Ba\TiØ_3$ based on data obtained from first-principles calculations within density functional theory. The equilibrium point defect concentrations are obtained via a self-consistent determination of the electron chemical potential. This allows to derive charge carrier concentrations for a given temperature and chemical environment and eventually the electrial conductivity. The calculations are in excellent agreement with experimental data if an accidental acceptor dopant level of $10^{17}\,\cm^{-3}$ is assumed. It is shown that doubly charged oxygen vacancies are accountable for the high-temperature $n$-type conduction under oxygen-poor conditions. The high-temperature $p$-type conduction observed at large oxygen pressures is due to barium vacancies and titanium-oxygen di-vacancies under Ti and Ba-rich conditions, respectively. Finally, the connection between the present approach and the mass-action law approach to point defect thermodynamics is discussed.

cond-mat.mtrl-sci↗

arXiv2010

Association of oxygen vacancies with impurity metal ions in lead titanate

Thermodynamic, structural and electronic properties of isolated copper and iron atoms as well as their complexes with oxygen vacancies in tetragonal lead titanate are investigated by means of first principles calculations. Both dopants exhibit a strong chemical driving force for the formation of M_Ti-V_O (M = Cu, Fe) defect associates. The most stable configurations corresponds to a local dipole aligned along the tetragonal axis parallel to the spontaneous polarization. Local spin moments are obtained and the calculated spin densities are discussed. The calculations provide a simple and consistent explanation for the experimental findings. The results are discussed in the context of models for degradation of ferroelectric materials.

cond-mat.mtrl-sci↗